Chaetopterus eggs have been studied during cleavage with cytochemical methods and the electron microscope. When the nucleus breaks down in prophase the nuclear contents show an increased RNA staining, PAS reaction, and a heightened concentration of amorphous and particulate dense material in electron micrographs. The nucleolus remains intact during these changes, which take about two minutes or less. The bilaminar membranous elements of the nuclear envelope pinch off at irregular intervals to produce vesicles in the plane of the nuclear surface. Secondarily the envelope vesicles leave the nuclear surface and wander into both the nuclear and cytoplasmic areas. The vesicles are morphologically indistinguishable from the characteristic general vesicles of the cytoplasm. In telophase, karyomeres fuse to form the daughter nuclei. The newly formed karyomeres show strong RNA staining in their interiors which disappears as the karyomeres “mature”. The reforming nuclear envelope is formed by a collection of general vesicles on the karyomere surface. The vesicles flatten out and fuse to form the usual bilaminar membranous elements. Annulated “pores” can be seen in the reforming envelope, having the characteristic dimensions even at the earliest stages. Arguments are presented for the view that the pore complex structure of the envelope is of nuclear origin; that the overall configuration of the envelope reflects the outline shape of the nuclear structure.